Refactoring: Font-lock code and section title handling functions share
functions. Also fixes problems with font-locking under `jit-lock-mode`.
`jit-lock-mode` is kept as `font-lock-support-mode`. Support for
`jit-lock-mode` has been debugged. For big blocks of comments and
literal text using `jit-lock-mode` may result in slowing display down.
Further work is needed here.
Added, removed and adapted tests.
Added ideas.
git-svn-id: https://docutils.svn.sourceforge.net/svnroot/docutils/trunk@6646 929543f6-e4f2-0310-98a6-ba3bd3dd1d04